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SubjectRe: [PATCH 0/6] musb: Improve performance for hub-attached webcams

ср, 3 апр. 2019 г. в 21:53, Matwey V. Kornilov <>:
> The series is concerned to issues with isochronous transfer while
> streaming the USB webcam data. I discovered the issue first time
> when attached PWC USB webcam to AM335x-based BeagleBone Black SBC.
> It appeared that the root issue was in numerous missed IN requests
> during isochronous transfer where each missing leaded to the frame
> drop. Since every IN request is triggered in MUSB driver
> individually, it is important to queue the send IN request as
> earlier as possible when the previous IN completed. At the same
> time the URB giveback handler of the device driver has also to be
> called there, that leads to arbitrarily delay depending on the
> device driver performance. The details with the references are
> described in [1].
> The issue has two parts:
> 1) peripheral driver URB callback performance
> 2) MUSB host driver performance
> It appeared that the first part is related to the wrong memory
> allocation strategy in the most USB webcam drivers. Non-cached
> memory is used in assumption that coherent DMA memory leads to
> the better performance than non-coherent memory in conjunction with
> the proper synchronization. Yet the assumption might be valid for
> x86 platforms some time ago, the issue was fixed for PWC driver in:
> 1161db6776bd ("media: usb: pwc: Don't use coherent DMA buffers for ISO transfer")
> that leads to 3.5x performance gain. The more generic fix for this
> common issue are coming for the rest drivers [2].
> The patch allowed successfully running full-speed USB PWC webcams
> attached directly to BeagleBone Black USB port.
> However, the second part of the issue is still present for
> peripheral device attached through the high-speed USB hub due to
> its 125us frame time. The patch series is intended to reorganize
> musb_advance_schedule() to allow host to send IN request quicker.
> The patch series is organized as the following. First three patches
> improve readability of the existing code in
> musb_advance_schedule(). Patches 4 and 5 introduce updated
> signature for musb_start_urb(). The last patch introduce new
> code-path in musb_advance_schedule() which allows for faster
> response.
> References:
> [1]
> [2]
> Matwey V. Kornilov (6):
> usb: musb: Use USB_DIR_IN when calling musb_advance_schedule()
> usb: musb: Introduce musb_qh_empty() helper function
> usb: musb: Introduce musb_qh_free() helper function
> usb: musb: Rename musb_start_urb() to musb_start_next_urb()
> usb: musb: Introduce musb_start_urb()
> usb: musb: Decrease URB starting latency in musb_advance_schedule()
> drivers/usb/musb/musb_host.c | 114 +++++++++++++++++++++++++++----------------
> 1 file changed, 71 insertions(+), 43 deletions(-)
> --
> 2.16.4

With best regards,
Matwey V. Kornilov

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